Dibenzyl trisulfide (DBTS) is a biologically active polysulfide secondary metabolite isolated from the sub-tropical shrub, Petiveria alliacea, L. (Phytolaccaceae) that has been reported to have immunomodulatory and anti-proliferative activity. See, e.g., Rosner et al., Biochim. Biophys. Acta, (2001), 1540:166-177; and Mata -Greenwood, et al. Anticancer Res. (2001), 21:1763-1770.
Further optimization of this compound led to the discovery of a new anticancer drug candidate, fluorapacin, bis(4-fluorobenzyl)trisulfide. See, e.g., An et al., Bioorg. Med. Chem. Lett. (2006), 16:4826-4829. The therapeutic application of fluorapacin as an anticancer drug has been previously described. See, e.g., PCT/US2005/013474 (International Publication WO 2005/112,933); U.S. Ser. No. 11/110,203 (published as US 2005/0261321, now allowed); and CP200580012460.5.
The naturally-occurring antibiotics varacin, lessoclinotoxin A, Calicheamicin and esperamicin derivatives include cyclic or acyclic polysulfide moieties which are critical for biological activities of these natural products. The synthesis of some small molecule polysulfide derivatives has been previously reported. See, e.g., Clennan & Stensaas, Org. Prep. Proc. Int. (1998), 30:551-600. However, the methods have seldom been used for the synthesis of biologically active and therapeutically useful compounds.
Harpp and co-workers reported the synthesis of dibenzyl trisulfide (Harpp et al., Tetrahedron Lett. (1970), 3551-3554) using the following scheme:

This synthetic route involved several steps and the use of unstable intermediates which resulted in low overall yields.
The same researchers also reported the synthesis of trisulfide derivatives (Harpp & Granata, Tetrahedron Lett. (1976), 3001-3004) using the following route:

The starting material for this synthesis, CCl3—SCl, is not commercially available and this reagent, as well as the two synthetic intermediates indicated in brackets, is unstable and toxic. Because of difficulty storing and handling the starting material and the intermediates, this route does not appear amenable to large scale synthesis.
The preparation of trisulfides by reaction of diimidazolylsulfide and thiols was described by Banerji & Kalena, Tetrahedron Lett. (1980), 21:3003-3004. A modification of this approach was described for the synthesis of trisulfide derivatives, including fluorapacin, on laboratory scale in U.S. Ser. No. 11/110,203.
Roy and co-workers recently reported the synthesis of symmetrical trisulfide derivatives using copper (II) salts and elemental sulfur (Sinha et al., Organometallics, (2001), 20:157-162) by the following route:

This route produced a complicated mixture of di-, tri-, tetra- and penta-sulfide derivatives, that were difficult to separate from the mixture because of the similar physicochemical properties of these derivatives. In preliminary studies using this approach and a model compound, it proved impossible to isolate the desired trisulfide from the mixture even utilizing HPLC. The partially purified trisulfide product also underwent disproportionation during and after the purification because of the existence of other impurities and polysulfides. Therefore, this approach could not be applied to the large scale synthesis of fluorapacin and related trisulfide derivatives.
Organo-sulfur compounds, and in particular sulfur chloride (—S—Cl) compounds, have a strong, irritating smell, and are difficult to synthesize. Therefore, the synthesis of organo-sulfur compounds, especially trisulfide derivatives, and their applications in biology, medicine, and other areas have been severely limited. Up to the present, no process has been reported in the literature for the large scale synthesis of trisulfide derivatives.
While a number of workers have reported methods for the preparation of trisulfide derivatives, these routes generally require multiple synthetic steps and/or the formation of unstable, toxic intermediates or starting materials. In addition, many of the literature routes are complicated by the formation of mixtures of products or by-products, making the desired trisulfide derivatives difficult to isolate and purify in good yields. The trisulfide products obtained are frequently unstable when not in highly pure form, and have been observed to undergo decomposition or disproportionation during and after purification due to the presence of impurities. Accordingly, there remains a need in the art for processes suitable for the large scale synthesis and purification of this novel class of biologically active molecule, especially ones that are to be used as drugs, where purity and stability are critical.
Therefore, it is essential to discover and develop an efficient and practical synthetic and manufacturing process for the trisulfides so that new anticancer drug fluorapacin and related trisulfide derivatives can be manufactured in large scale to fulfill the clinical and further development needs. The discovery and development successes of the new anticancer drug fluorapacin [bis(4-fluorobenzyl)trisulfide] also encouraged us to develop an efficient, practical, inexpensive, and safe technological process for large scale synthesis and manufacture of trisulfide derivatives.